Hydrogen is utilized in a variety of industrial processes and is typically produced from the steam methane reforming of hydrocarbons that are contained in natural gas. The hydrogen after production can be used on site where the steam methane reformer is located or can be distributed to customers with the use of a pipeline.
Customer demand will typically vary and it has been found to be advantageous to store the hydrogen when customer demand is low for later use during peak demand periods. One central advantage in storing the hydrogen is that the hydrogen production facility does not have to be sized to meet peak customer demands and thus, can be a smaller, lower cost installation. Additionally, the storage of hydrogen also allows for further profitability in that spot sales of hydrogen can be made to customers, above that required to meet contractual customer demands. Since the storage requirements related to pipeline distribution systems can be at a level of over a billion standard cubic feet of hydrogen, large underground geological formations known as salt caverns are used for such purposes.
Salt caverns are formed by solution mining within large underground formations of salt that can be several hundred to several thousands of feet deep that are generally covered by a cap rock. In order to form the salt cavern, a well is drilled from the surface down to and through the salt formation. The well hole is supported by pipe casings which are surrounded by concrete and a smaller pipe, known as a brine string, is inserted into the casing to introduce water down into the hole. The water dissolves the salt to form the cavern and the resulting brine is pumped to the surface between the annular space formed between the casing and the smaller pipe. Additionally, brine is removed from the cavern by injecting hydrogen to pressurize the cavern and force the brine out through the brine string. When complete, the salt cavern has a roof region at the top of the salt cavern and beneath the cap rock, side regions connecting the roof regions with the bottom of the salt cavern and a residual brine layer or brine sump remaining at the bottom of the salt cavern.
The hydrogen to be stored can be produced at the site of the salt cavern or can be removed from the pipeline itself. The hydrogen is compressed by a compressor and the resulting compressed hydrogen feed stream is introduced into the salt cavern through the casing. When the hydrogen required to meet customer demand is greater than that able to be produced by the hydrogen production facility, the hydrogen is taken from the salt cavern as a stored hydrogen stream through the casings and injected back into the pipeline.
Carbon dioxide as well as other impurities can be introduced into the stored hydrogen stream from the salt cavern itself. The carbon dioxide impurity, as well as moisture, for the most part, evolves from the residual brine layer. Since the pipeline itself will have a specification for the amount of carbon dioxide and also, possibly moisture, that can be contained in the hydrogen that is reintroduced into the pipeline, the carbon dioxide impurity that is introduced into the stored hydrogen stream is removed by an adsorption unit that contains an adsorbent to adsorb the carbon dioxide and also possibly the moisture. For example, in U.S. Pat. No. 7,078,011, a purification system is used in connection with the salt cavern to reduce levels of carbon dioxide and moisture to sufficiently low levels that are necessary to meet the pipeline specification. It has been found that the use of carbon dioxide purification system adds a level of complexity and expense to the operation of an installation that involves the storage of hydrogen within a salt cavern. As will be discussed, it has been found that although unacceptably high levels of carbon dioxide can initially be imparted from the salt cavern to the stored hydrogen stream to be injected back into the pipeline, when the salt cavern is operated in a manner as described in the present invention, carbon dioxide removal will not be necessary.